Functional connections between auditory cortex on Heschl's gyrus and on the lateral superior temporal gyrus in humans

Functional connections between auditory fields on Heschl's gyrus (HG) and the acoustically responsive posterior lateral superior temporal gyrus (field PLST) were studied using electrical stimulation and recording methods in patients undergoing diagnosis and treatment of intractable epilepsy. Averaged auditory (click-train) evoked potentials were recorded from multicontact subdural recording arrays chronically implanted over the lateral surface of the superior temporal gyrus (STG) and from modified depth electrodes inserted into HG. Biphasic electrical pulses (bipolar, constant current, 0.2 ms) were delivered to HG sites while recording from the electrode array over acoustically responsive STG cortex. Stimulation of sites along the mediolateral extent of HG resulted in complex waveforms distributed over posterolateral STG. These areas overlapped each other and field PLST. For any given HG stimulus site, the morphology of the electrically evoked waveform varied across the STG map. A characteristic waveform was recorded at the site of maximal amplitude of response to stimulation of mesial HG [presumed primary auditory field (AI)]. Latency measurements suggest that the earliest evoked wave resulted from activation of connections within the cortex. Waveforms changed with changes in rate of electrical HG stimulation or with shifts in the HG stimulus site. Data suggest widespread convergence and divergence of input from HG to posterior STG. Evidence is presented for a reciprocal functional projection, from posterolateral STG to HG. Results indicate that in humans there is a processing stream from AI on mesial HG to an associational auditory field (PLST) on the lateral surface of the superior temporal gyrus.